Tags can be used in many different ways. Many users have asked for sub-projects, sub-task, or just a simple way to track phases of their projects. You can most definitely use tags in this way.
Tags come two different ways: Tags and Subtags.
These are the main tags. Meaning they are the main task you're working on within a project. You can add as many of them as you need or want to an entry.
These are secondary tags or tasks to a project. Many would argue they are great in instances such as a phase on a project.
Note: You can only add one subtag under a main tag to an entry.
If you have another way you use tags do tell so we can let others know too!